Revisiting an Early Devonian bony fish: Doliodus

Over the past few weeks
I’ve been revisiting the 82 taxa currently in the ray-fin fish clade. Many were entered into the large reptile tree (LRT, 1793+ taxa) while I was still a ‘freshman’ in fish morphology. So it is no surprise that corrections are past due.

As an example,
earlier the nesting of swordfish with European eels and Bavaricthys was only a preview of what was to come. The work is never done.

Longtime readers know
making additions and corrections has been an ongoing process for the last ten years of this online study. The number of corrections made during that time is deep into six figures.

Today let’s revisit 
Doliodus problematicus (Whiteaves 1881; Traquair 1893; Miller, Cloutier and Turner 2003; Maisey et al. 2018; Early Devonian) a crushed skull with strange double-pronged teeth (Fig. 1). It was recently considered the ‘oldest articulated chondrichthyan’ and a transitional taxon between acanthodians and sharks. That is not confirmed in the LRT.

Figure 1. Doliodus from Maisey et al. 2018. Colors added. Compared to Malacosteus in figure 2.

Figure 1. Doliodus from Maisey et al. 2018. Colors added. Compared to Malacosteus in figure 2.

Doliodus nests with the extant, deep-sea, big-mouth, basal bony fish. Malacosteus (Fig. 2) in the LRT (Fig. x). The skull of Doliodus is wider, the maxilla is flat and the dual-pronged teeth are oriented medially (shown ventrally here, Fig. 1, for clarity). Considering this relationship, Doliodus might have been a nocturnal or deep-sea feeder. Convergent with spiny sharks, the leading edge of the Doliodus pectoral fin was a robust spine.

Figure 2. Malacosteus µCT scan from Kenaley 2007. Colors added. Compare to Doliodus in figure 1.

Figure 2. Malacosteus µCT scan from Kenaley 2007. Colors added. Compare to Doliodus in figure 1.

The stoplight loosejaw, a type of deep sea dragonfish,
Malacosteus niger 
(Ayres 1848, 1849; Kenaley 2007; 25cm; Fig. 2) is now a sister to Dolidodus in the LRT.  The eyeball is at the extreme anterior of the skull. The rostrum is minimized. The jaw joint is far behind. No operculum is present. The posterior dorsal and anal fins are much larger than the caudal fin. Light emitting organs are present posterior to the eye.

Figure x. Subset of the LRT focusing on ray-fin fish and the ancestry of flatfish. Note Scorpis and Polydactylus are close to flatfish, but Pomatomus is closer.

Figure x. Figure x. Current subset of the LRT focusing on ray-fin fish.

According to Kenaley 2007, 
“This genus is diagnosed within the Stomiidae by having enormous jaws, a single circular nostril on each side of the snout, a large tear-shaped accessory orbital photophore, serial photophores reduced in size and number, and intramandibular membrane, hyoid barbel, and palatine teeth absent.”

Kenaley 2007 does not mention Doliodus. Maisey et al. 2018 do not mention Malacosteus. So this (Fig. x) may be a novel hypothesis of interrelationships. If not, please provide a prior citation so I can promote it.

Figure 3. Malacosteus niger in lateral view.

Figure 3. Malacosteus niger in lateral view.

Taxon inclusion and comparative anatomy
are the keys to understanding vertebrate interrelationships.

Ayres WO 1848. pp. 64–73. In: Proceedings of the Boston Society of Natural History, Vol. 3. Proceedings of the Boston Society of Natural History, Boston.
Ayres WO 1849. Description of a new genus of fishes, Malacosteus. Boston Journal of Natural History 6:53–64.
Kenaley CP 2007. Revision of the Stoplight Loosejaw Genus Malacosteus (Teleostei: Stomiidae: Malacosteinae), with Description of a New Species from the Temperate Southern Hemisphere and Indian Ocean. Copeia. 2007 (4): 886–900. doi:10.1643/0045-8511(2007)7[886:
Maisey JG et al. (6 co-authors) 2018. Doliodus and Pucapampellids: Contrasting perspectives on stem chondrichthyan morphology. Chapter 5 in Evolution and Development of Fishes.
Miller RF, Cloutier R and Turner S 2003. The oldest articulated chondrichthyan from the Early Devonian period. Nature 435:501–504.
Traquair RH 1893. Notes on the Devonian fishes of Campbellton and Scaumenac Bay in Canada. No. 2. Geological Magazine, decade 3, 10: 145–149.
Turner S and Miller RF 2004. New ideas about old sharks. American Scientist 93:244–252.
Whiteaves JF. 1881. On some fossil fishes, Crustacea and Mollusca from the Devonian rocks at Campbellton, NB, with descriptions of five new species. Can Nat 10:93–101.

wiki/Doliodus not yet published


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